In view of safety at the time of a vehicle collision, a known seat apparatus for a vehicle is equipped with a mechanism for moving a headrest in a vehicle's forward direction for the purposes of protecting the head of a seated occupant in the event of a rear end collision (i.e., headrest apparatus for a vehicle). Such apparatus is disclosed in JP 2007-030676A and JP 2007-168752A. According to each of the headrest apparatuses disclosed, a headrest is moved in the forward direction relative to a seatback, thereby protecting the head of the seated occupant. Thus, in the event of or immediately before a rear end collision, an impact applied to the neck of the occupant may be decreased.
In this case, a capacitance sensor is used for stopping the movement of the headrest in response to the position of the occupant's head. For example, as shown in FIG. 2A, the capacitance sensor is provided at a front portion of the headrest (specifically, a surface facing the head of the occupant) for detecting a variation in capacitance that occurs in association with an approach of a detection object such as the head of an occupant to the front portion of the headrest. Then, the approach of the head of the occupant is determined on the basis of the variation in capacitance of the capacitance sensor. When the headrest approaches the head of the occupant as illustrated in FIG. 2B, a distance between the head and the headrest is shortened, which leads to an increase of a capacitance value of the capacitance sensor. As a result, it is detected that the headrest is positioned close to the head of the occupant.
The aforementioned headrest apparatus, which aims to protect the occupant in the vehicle, is required to surely detect the head of the occupant while the headrest is moved in the vehicle forward direction. In order to detect the head of the occupant, the capacitance sensor is oscillated via a drive signal with a high frequency in response to a command from a control unit so that a magnetic field is generated from the capacitance sensor in a direction where the head is positioned. Then, a variation in capacitance between the capacitance sensor and the detection object, i.e., the head of the occupant, is detected by a capacitance detection circuit.
The headrest apparatus is also desirably provided at rear seats in the vehicle so that the occupants not only in the front seats but also in the rear seats are protected in the event of or immediately before the rear end collision, thereby enhancing the safety. In this case, a distance between the headrest apparatus provided at the rear seat and a window provided at a rear portion of the vehicle is relatively short.
Lately, in order to improve the receiver sensitivity of a radio provided in the vehicle, a radio antenna is arranged in a window glass at the rear portion of the vehicle. In this case, when the headrest apparatus is provided at the rear seat, problems such as described below may occur. That is, when the capacitance sensor is driven via a drive signal with a high frequency, a noise is emitted outward from a line that drives the capacitance sensor (i.e., sensor line). This noise enters through the radio antenna arranged at the rear portion of the vehicle, which may cause discomfort to the occupant when he/she listens to the radio.
In order to decrease such radio noise, an operation time of the capacitance sensor may be minimized or a frequency of the drive signal for the capacitance sensor may be lowered as compared to a conventional drive frequency (for example, from several MHz to several KHz).
However, if the drive frequency is reduced as compared to the conventional one, the detection performance of the head of the occupant may be affected. The distance between the head and the capacitance sensor may be prevented from being surely detected, thereby causing the decrease in the detection performance. In this case, if a low-pass filter is simply conducted on the capacitance value detected by the capacitance sensor, the detection performance may decrease. As a result, countermeasures are required for the accurate detection of the detection object.
A need thus exists for a headrest apparatus for a vehicle which is not susceptible to the drawback mentioned above.